Bying negative amounts of cargo makes a profit

[GoogleCodeExporter]

When buying cargo you can enter "-1000" and you do end up buying minus one
thousand items. 

If you've got an empty hold and no cash, buying -1000 lots of food at 100cr
gives you 100,000 credits and -1,000 free cargo bays. 

NB: I've made this a Priority-Medium (rather than a Priority-Low) because
the Qwerty keyboard used for text entry makes it too easy to hit "-5"
instead of "15" due to how the keyboard is laid out.

(If the dialog is changed to use a number pad then this can become a low
and postponed if necessary.)

Original issue reported on code.google.com by RussellB...@gmail.com on 9 Jun 2009 at 9:25